Pre-rounding element on a rounding apparatus

ABSTRACT

A pre-rounding element ( 14 ) is used in a rounding apparatus ( 4 ) for rounding can body blanks from sheet metal blanks. The lower element part ( 22 ) of the pre-rounding element ( 14 ) is displaceable by an actuation element ( 23 ). This allows the adjustment of the pre-rounding element even during operation of the rounding apparatus. Preferably a measurement of sheet metal blank properties takes place. The measurement may include a thickness measurement and/or a strength measurement of blanks. The pre-rounding element is controlled in dependency of the measurement result. This allows a constant rounding result even with varying sheet metal properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 12/425,663, filed Apr. 17, 2009 and claims the priority ofSwiss patent application No. 00608/08, filed Apr. 18, 2008, thedisclosures of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

The invention relates to a pre-rounding element on a rounding apparatus,further to a rounding apparatus, to a welding device for can bodiesincluding such a rounding apparatus, as well as to a method for roundingsingle sheet metal blanks to container body blanks and to a method formaking can bodies from single sheet metal blanks.

PRIOR ART

It is known to use a rounding apparatus for the making of containerbodies and in particular can bodies from sheet metal. After the roundingthe so produced container body blanks are directly transported into awelding machine for welding of the seam of the container body. Thede-stacker of the sheet metal blanks, the rounding apparatus and thewelding machine can be arranged as one production line. Such productionlines for the making of can bodies are for example known from DE-A-33 30171 or from U.S. Pat. No. 5,209,625. The rounding is done in such amanner that the can body blank is fed directly into the so called Z-raildefining the overlap of the edges of the rounded blank. For therounding, rectangular cut sheet metal blanks with defined dimensions andstandardised properties of the metal sheets are pushed by a feeder intoa first pair of driven transport rollers and are then transported byseveral driven transport rollers with a speed of 100 to 450 meters perminute and bent in a rounding apparatus with a rounding system withwedges and rollers or with a roller system to a round body. Inparticular a pre-rounding wedge is used in front of the roundingrollers, which influences the result of the rounding procedure. In frontof the rounding station there may be provided a wedge-system of anoptional flexing station providing a plastic deformation of the blankthat serves for removing tensions of the sheet metal blank before therounding process. Such rounding apparatus or rounding devices,respectively, are known to the skilled person. The sheet metal blanksused for series production have different thickness and properties asfor example elastic limit, stress-strain characteristic and strainhardening properties, which after the rounding process lead to differentbody radii depending on the qualities of the sheet metal which in turnlead to different gaps between the free edges. Thus not all roundedbodies of the production series lie in the same position within therounding station and are exhibiting different rounding radii and thismay lead to a variation of the size of the edge overlap in the weldingstation which leads to problems during welding, or may lead to problemsalready when the bodies are laterally pushed out from the roundingstation into the welding station. This may lead on the other hand to amachine stop with considerable down-times. The efficiency of theproduction line is thus reduced and down-time costs occur.

According to the prior art the adjustment of the pre-rounding wedge isdone manually and depends on the mentioned sheet metal properties suchas the elastic limit and sheet thickness as well as from the size of thesheet metal blanks. If production is run with different sheet metalqualities it is necessary to adjust the pre-rounding wedge each timewhile the rounding apparatus is stopped.

To avoid these problems during rounding and welding of can bodies it isknown to use sheet metal blanks with only little variations of the sheetmetal properties coming from a single production charge of the sheetmetal maker. It is taken care that sheet metal with different propertiesis not mixed-up. The rounding result is checked often and the roundingsystem and in particular the pre-rounding wedge is readjusted, which isdone when the machine is stopped. But this reduces the efficiency of theproduction line.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to overcome these drawbacks.

Now, in order to implement these and still further objects of theinvention, which will become more readily apparent as the descriptionproceeds, a pre-rounding element according to claim 1 is provided thatallows changing the pressurization by the pre-rounding element of thesheet metal blank to be rounded during the actual operation of therounding apparatus.

Preferably, the pre-rounding element, in particular a pre-roundingwedge, is provided with a solid flexure hinge, resulting in a simpleconstruction. The solid flexure hinge works without unwanted play andfree from wear. It is preferred that the pre-rounding element isprovided with an electrically controllable actuation-element, resultingin a simple control and a low reaction time for changing thepressurization on the sheet metal blank.

The pre-rounding element can be controlled by an input value or by apreset value of the control device of the rounding apparatus, so that anadaptation to different sheet metal properties can be done by the workercontrolling the rounding apparatus, or that for example a preset valuefor the setting of the pre-rounding element can be called up by theinput of sheet metal blank properties such as blank size and/or blankthickness. It is preferred on the other hand that a measuring value of asheet metal property serves for setting the pre-rounding element. It isparticularly preferred that a property of the blanks is measured duringrounding operation and that the pre-rounding element is set duringoperation of the rounding apparatus according to the measured property.

With a rounding apparatus according to the invention and a weldingdevice according to the invention and with the method of rounding andthe method of welding, respectively, such a pre-rounding element isused. It is provided in particular at least one measuring device formeasuring at least one sheet metal property so that a measurement valueor a value derived from this measurement value is given to a control ofthe rounding apparatus for controlling the pre-rounding element of therounding apparatus in dependency of the measurement value or the valuethat is derived from the measurement value. This results in a roundingradius that remains essentially constant despite sheet metal propertychanges.

In a preferred method or device, respectively, the measurement is madeduring the rounding operation and without sheet metal blank destructionon consecutive sheet metal blanks, so that measurement takes placeduring production and the pre-rounding is set during production. Therounding is made during the production of can bodies, and in particularwith a rounding speed of 100 to 450 meters per minute, and the roundedcan body blanks are fed from the rounding apparatus to a welding devicefor can body blanks, provided with a Z-rail for positioning the edges tobe welded and provided with welding rollers, and in particular with wireelectrodes running on the welding rollers. The thickness of the sheetmetal blank is an example for the sheet metal property that is measured.

But preferably, the sheet metal property that is measured is directlythe rounding characteristic or the elastic limit, respectively, so thata direct value for the rounding property of a blank or even of eachblank is provided. This can preferably be done by executing a partialpre-rounding in the feed path to the rounding apparatus or within therounding apparatus itself. In particular, this can be provided withinthe flexing station and in particular the rounding property will bedetermined electrically and/or mechanically and/or optically and/oracoustically.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those setforth above will become apparent when consideration is given to thedependent claims and to the following detailed description of preferredembodiments. Such description makes reference to the annexed drawings,wherein

FIG. 1 shows a side-view of an embodiment of a pre-rounding elementaccording to the invention;

FIG. 2 shows schematically the arrangement of a pre-rounding elementaccording to the invention within a rounding apparatus;

FIGS. 3 to 11 show schematically other embodiments of pre-roundingelements;

FIG. 12 shows schematically a device for explaining embodiments of theinvention;

FIG. 13 shows another device;

FIG. 14 shows a schematic view of a measurement device;

FIG. 15 shows a perspective partial view of the measurement device ofFIG. 14; and

FIG. 16 shows the electrical circuit of the measurement device of FIGS.14 and 15.

WAYS OF IMPLEMENTING THE INVENTION

FIG. 1 shows a side view of an embodiment of a pre-rounding element 14according to the invention. The pressurization of a sheet metal blankwithin a rounding apparatus by such a pre-rounding element 14 isschematically shown in FIG. 2. A more detailed description of a roundingapparatus with a pre-rounding element and with measurement of sheetmetal blank properties is given with respect to FIGS. 12 to 16.

The pre-rounding element 14 is in the shown embodiment a wedge-shapedpre-rounding element or a pre-rounding wedge, respectively, since itslower part 22 which acts on the sheet metal blank has a wedge-shape. Theacting on the sheet metal blank by the pre-rounding wedge leads to apressure acting on the blank and is thus here called “pressurization”.The pre-rounding element comprises a rigid, fixedly mounted upper part20. In this embodiment part 20 is a part adapted to be mounted on amachine part of the rounding apparatus and is provided in the shownexample with a bore 26 and guides 24 allowing fixation to the roundingapparatus and as well providing for an adjustment in vertical direction.Thus a basic adjustment can be made when mounting the pre-roundingelement to the rounding apparatus. The fixed upper part 20 may as wellbe a rigid part of the rounding apparatus itself.

The movable lower part 22 or wedge 22, respectively, of the pre-roundingelement 14 is connected to the fixed upper part 20 by a joint or hinge,respectively, which allows the movement or displacement, respectively,of the lower part 22, relative to the fixed upper part. This movement ordisplacement is shown by arrow a. The movement or displacement,respectively, is provided by an actuation element 23, which is alsocalled an actor. The actor has a non-moving upper part which is fixed atthe upper part 20 of the element 14 and the movable part 23′ of theactor acts on the displaceable wedge-shaped lower part 22 of thepre-rounding element 14. Thus if the movable part 23′ acts with a forcein downward direction (when the downward direction is defined as seen inthe Figure) the movable wedge-shaped part 22 is moved or displaced,respectively, so that it will assume in its maximum bent position theposition shown by the dotted line. If on the other hand the movable part23′ of the actor 23 is in its rest position (exerting no force on thewedge) the wedge-shaped part 22 is in the rest position shown by thefull line. Depending on the force with which the actor acts on thewedge-shaped part 22 a position between the two shown positions results.The joint or hinge 21, respectively, which connects the upper part 20and the wedge 22 is in this case a flexible solid hinge consisting of asolid part with a dimension that allows deformation within its elasticrange by the force of the actor 23. Accordingly a movement back towardsthe normal position (which is the position without a force acting on thehinge) results, when the force of the actuator is reduced. Such a hingeor joint 21 being in one-piece with the upper part 20 and the lower part22 as shown in FIG. 1 is the preferred embodiment. The actor can be acontrollable drive of any construction that allows deflection of thewedge. A known piezoelectric actor is the preferred actor since onlysmall deflections of the wedge are necessary to act on the sheet metalblank and the quick response of a piezoelectric actor is an advantage.But other controllable drives such as electric motors, magnetic drivesor a pneumatic or hydraulic drive may be used as actor 23.

FIG. 2 shows schematically the feeding of a sheet metal blank 1 towardsa rounding apparatus 4 in which another sheet metal blank 2 is alreadybeing rounded. The sheet metal blanks have been de-stacked from a stacknot shown here and are rounded while travelling through the roundingapparatus. The transport of a de-stacked blank into the roundingapparatus is provided by any kind of known transport elements, forexample by the rollers shown. A flexing-station with flexing rollers 8and 9 and with a flexing wedge 7 may be provided. Such flexing stationsare known and on the one hand remove tensions from the blank which helpsthe following rounding process and on the other hand provide a roundingof the foremost edge of each blank which is helpful for a perfectrounding result. A measurement unit as will be explained later in detailmay be present as part of the flexing station or may follow directlyafter the flexing station, in particular for measuring the roundingproperties of each blank. Following the flexing station the actualrounding station 4 of the rounding apparatus is schematically shown.This rounding station 4 includes rounding rollers 11 and 12 and arounding wedge 13 following in transport direction after the roundingrollers. These elements of a rounding apparatus are well known to theskilled person and are not explained here. In front of the roundingrollers 11, 12, the pre-rounding element 14 according to the inventionis arranged and acts on the sheet metal blank to be rounded. This isdone according to the invention by the displaceable lower part 22 of thepre-rounding element 14 during the rounding operation of the productionline. Thus it becomes possible to act by the element 14 during therounding operation individually on each single blank that travelsthrough the rounding apparatus and the rounding result can be influencedfor each single blank. The actor 23 is controlled in this respect by asignal from a control 5. This may be the control of the roundingapparatus or the control of a welding device for container bodies whichincludes the rounding apparatus. The control 5 may be as well a separatecontrol which may work together with the mentioned controls. The settingof the pre-rounding element 14 via control 5 may be done by a operatingperson operating the rounding apparatus for example by selecting aspecific displacement of the lower part 22 via control 5. Thisdisplacement is then used for all blanks until the operating personselects a different displacement. The displacement may also be selectedby the control 5 from a number of stored displacement values. Theoperating person then only enters the sheet metal blank size and/orsheet metal blank thickness and/or a code value for the sheet metal tobe rounded and the control selects the corresponding displacement of thelower part 22. In another and preferred embodiment a measurement of atleast one sheet metal property is performed by a measurement device 27and the measured value leads to a selection of the displacement of thelower part 22 by the control 5 via actor 23. The measurement ispreferably done for each blank and thus the pre-rounding element isindividually set for each blank of a series of blanks to be rounded. Themeasurement may as well take place for selected blanks, for example forthe 10^(th) and the 20^(th) and the 30^(th) blank and so on or only fora single blank of a number of blanks, for example for the first blank ofeach stack of blanks that is to be rounded. As a measured value thethickness of the sheet metal may be measured. Thickness measurementdevices for sheet metal are well known to the skilled person and aretherefore not explained here in detail. But preferably the actualrounding property of the sheet metal is measured and used forcontrolling the pre-rounding element 14, which is explained in detailbelow.

FIGS. 3 to 11 show other embodiments of the pre-rounding element 14. Inthese Figures the actor is not shown as an element but only the force Fof the actor acting on the displaceable part of the pre-rounding elementis represented by an arrow. According to FIG. 3 the pre-rounding elementis shown with an elastically solid joint 21 provided by at least oneleaf spring. As the solid joint of FIG. 1 it provides for an automaticmovement back to the rest position when the force of the actor ends.FIG. 4 on the other hand shows a rotating joint 21 with a rotationalaxle 21′. In this case the movement back to the rest position has to bedone by the actor or by another element not shown, for example a backmovement spring or by the counter force of the sheet metal blank itselfacting on the lower part 22. FIG. 5 shows an embodiment with a jointthat allows for a rocker-like movement of the lower element 22 by thetwo forces F (and thus by two actors). FIG. 6 shows another solid joint21 and shows the upper part 20 as a solid part of the roundingapparatus. This is shown as well in FIG. 7 where the joint or hinge,respectively, is provided by at least one leaf spring. The pre-roundingelements 14 of FIGS. 8 to 11 again are provided with one-piece solidjoints working by elastic deformation of parts of the pre-roundingelements 14. The terms upper element and lower element have been used,but of course, depending on the construction of the rounding apparatusit is possible, that the lower part of the pre-rounding element is thefixed part and the upper part is the part that is displaced by theactor.

FIGS. 12 and 13 schematically show different embodiments of the presentinvention wherein same reference numerals are used for the elementsalready explained. It can be seen, that sheet metal blanks, of whichblanks 1 and 2 are shown as examples, are de-stacked from a stack 10 (bya known de-stacker not shown) and are fed to a transport device 3 whichis used as a feeding element for a rounding apparatus 4 and provides afeeding section of a certain length. The blanks are transported throughthis arrangement of feeding element and rounding apparatus in directionof arrow A. The de-stacking and the feeding into the transport device 3are not explained here in detail since this is known to the skilledperson. Although preferred, the transport device 3 is only optional sothat the blanks may be fed directly from the stack into the roundingapparatus. This makes it necessary though that the measurement devicewhich is explained below is positioned at the entrance of the roundingapparatus or within the rounding apparatus, which will be shown as well.Such a positioning of the measurement device is of course as wellpossible if a transport device 3 is present. In the shown embodiment thetransport device is provided with several pairs of rollers whichtransport each blank to the entrance 25 of the rounding apparatus 4.Such a transport may be done in other ways known to the skilled personthan by the shown pairs of rollers. Within the rounding apparatus eachblank is rounded to a body blank as can be seen for the front part (seenin transport direction) of blank 2. The rounding is done with a settarget rounding radius that is given by the setting of the roundingapparatus and actually leads to a rounding radius R. The rounding isdone for example with a rounding speed of 100 meters per minute up to450 meters per minute. Different kinds of rounding apparatus are knownand available, in particular as well for can bodies. A simple embodimentof a rounding apparatus may use two rounding rollers 11 and 12 as shown.Rounding apparatus with more than two rounding rollers are known aswell, for example from EP-A-1 197 272. Such rounding apparatus and otherknown rounding apparatus can be used within the scope of the presentinvention as well. It is again shown that a pre-rounding element 14 ispresent in front of rounding rollers 11, 12, but this element 14 isshown in a simplified manner, but it is to be understood that theelement 14 is one of the embodiments explained above. A rounding wedge13 may be present behind rounding rollers 11, 12 as shown in FIG. 12,but such a rounding wedge may as well be omitted as shown in FIG. 13. Itis further possible and even preferred that a flexing station is presentin front of the actual rounding station, which flexing station is herepart of the rounding apparatus but may as well be a separate station. Inthe example shown the flexing station comprises rollers 8 and 9 and theflexing wedge 7 acting on the blank exiting from rollers 8, 9. Flexingstations are generally known to the skilled person for removing tensionsfrom the sheet metal and helping the following rounding, for examplefrom U.S. Pat. No. 5,209,625 mentioned above, and are not explainedfurther here; but the measurement device for detecting the roundingbehaviour or properties, respectively, may be provided as well in theflexing station and the pre-rounding in the flexing station may be usedfor detecting the rounding behaviour of the sheet metal, as will beexplained more in detail.

The adjustable elements of the rounding apparatus, and preferably aswell of the flexing station, are preferably provided with drives(hereinafter called actors as well) which can move these elements withintheir usual adjustability ranges for allowing the control of therounding apparatus to influence the rounding result; the influence ofthe actors on the elements of the rounding apparatus is symbolized inthe Figures by arrows leaving the actors and pointing to the elements.The movement or displacement of the element is symbolized by anotherarrow and the connection of the actors with the control 5 is symbolizedby lines 40. Thus the flexing wedge 7 can be moved by actor 6 in thedirections of arrow B. The pre-rounding wedge is moved by actor 23 inthe way already explained. For rollers 11 and 12 a drive may be providedthat adjusts the distance between the rollers and which may act on bothrollers or just on one of the rollers. This is shown schematically bythe actor 16. Further, an actor 17 may be provided that acts on therounding wedge 13 to move this rounding element as shown by arrow D. Allof these actors may be provided in addition to actor 23 or none of theseadditional actors may be provided or only one of these actors and anycombination thereof may be provided, thus allowing the control 5 of therounding apparatus 4 to influence the rounding result and the roundingradius R, respectively, directly during operation of the apparatus bycontrolling the actors and thus the rollers and/or wedges. The design ofthe actual drives and their linkage to the rollers and wedges may varyaccording to the actual construction of the rounding apparatus, but canbe easily adapted by the skilled person. The actors may work on aelectric, pneumatic, hydraulic or piezoelectric basis, for moving theelements of the rounding apparatus. Such movement shall be attainedduring the operation of the apparatus so that a change of the roundingradius can be effected from one blank to the next or even during therounding of one blank. The rounding apparatus may be operated in a basicoperation mode first that corresponds to the expected general propertiesof the blanks of the stack 10 which leads to the rounding radius R thatshall be attained as long as the blank properties are within theexpected range. If the measured properties deviate from this range, aswill be explained below, the control can activate at least one of theactors in dependency of the measurement value to adapt the roundingparameters to the measured blank properties, so that despite thedeviation of the properties the rounding result with the target value ofthe rounding radius R is attained. If only one of the actors is present,and thus in this case only actor 23, which acts on the pre-roundingelement 14, the control 5 can control this actor only and the controlcan be easily programmed by a few tests with sample blanks withdifferent properties, so that for these properties the correct result isattained. If then during operation the measurement shows that a blankwith a measured value that equals one of several stored values or lieswithin one of several stored ranges of measurement values, the controlcan react according to the test sample settings and set the pre-roundingelement accordingly. It can be seen that by providing more than oneactor and thus giving more than one possibility to the control to actagainst the deviation of the blank properties, the complexity of theprogram within control 5 rises, since for example, it has then to bedecided whether a change in blank properties is countered for attainingthe target value R by setting the pre-rounding element 14 by actor 23only or additionally by actor 16 setting the distance of roundingrollers 11 and 12. This as well can be selected beforehand by runningsample blanks through the rounding apparatus by the skilled personsetting the apparatus up for operation and setting and/or programmingcontroller 5. The same applies for the embodiment where the flexingwedge 7 can be displaced by an actor 6 as well. But since the effects onthe rounding result by elements 7, 14, 11 and 12 and if the case may be13, are already known to the skilled person for a rounding apparatus,this person can program the control 5 in such a way that the changes ofthe settings that this skilled person would have otherwise performedoffline and manually in view of a certain rounding result are now doneonline during operation of the rounding apparatus by the control and theactors.

According to embodiments of the invention a measurement device for thesheet metal blanks is provided by which at least one sheet metalproperty of the sheet metal to be rounded can be evaluated, so that theapparatus can be set accordingly. The invention includes the possibilitythat at least one blank of the stack 10 is measured before the roundingoperation on the blanks of this stack is started; this measurement maythen include a measurement by which the blank is destroyed. Based onsuch a measurement value the apparatus 4 and in particular thepre-rounding element is then set. A measurement device measuring beforethe start of the rounding operation may preferably be connected to thecontrol 5 over a data connection so that the control 5 receives themeasurement value on the blank properties of the stack directly, so thatactor 23, or if the case may be additional actors can be set. But thepreferred embodiment is to provide the measurement during the roundingoperation as explained below. In the shown embodiment of FIG. 12 themeasurement of at least one sheet metal property is done in the feedingsection to the rounding apparatus which is here provided by thetransport device 3. If such a feeding section is missing so that theblank is fed directly from stack 10 into the entrance 25 of the roundingapparatus 4, then the measurement of the at least one sheet metalproperty is done either at the de-stacker and/or directly at theentrance of or within the rounding apparatus 4, in particular in theflexing station. Such an example is shown in FIGS. 14 to 16. It is aswell a possibility to provide measurement devices at the blank slitterapparatus, which cuts large sheet metal pieces into the much smallersized blanks. In such an embodiment it is possible to mark the blankswith a marking, for example a number code or a bar code, whichrepresents the measured sheet metal blank properties, so that this codecan be read in the feeding section 3 or at the entrance of the roundingapparatus, which informs the control 5 about the measurement value orvalues or values derived from the measured values. This again allows thesetting of at least actor 23.

In the shown embodiment of FIG. 12 a measurement device 27 is shownwhich is arranged between pairs of rollers of the feeding section 3.This measurement device 27 is connected to the control 5, so that themeasured value or a value derived therefrom standing for the blankproperty or properties is given to control 5. Preferably the measurementdevice allows the measurement of the strength of the blank within themeasurement device. For example a measurement method is used that workswithout touching the blank. A known touch-less measurement device isused for steel bands and can here be used in a new manner for singleblanks. This method is based on a periodical magnetization of the metaland the measurement of the gradient of the remaining magnetic fieldstrength on the upper side and on the lower side of the band, but hereon these sides of the blank instead. The measured value of the remainingmagnetic field strength or the calculated gradient thereof,respectively, is related via correlation to the mechanical strength ofthe sheet metal, in particular to the tear strength and to the elasticlimit of the sheet metal blank. Such a measurement device is known underthe trademark IMPOC® and available on the market and is produced andsold by the company EMG Automation GmbH, Wenden, Germany. With such anmeasurement device the strength properties of the sheet metal blanks canbe detected which directly have an influence on the rounding properties,and the resulting measurement value will be given to the control 5 whichin particular upon an increase or a decrease of the measured strength incomparison to a set target strength value or value range operates atleast actor 23 to adapt the rounding apparatus during operation to thechange of the strength of the metal sheet blank. If thus the measuredstrength value for blank 1 deviates from a target value or target valuerange and reaches another target value or target value range for whichthe control has programmed orders for setting the rounding apparatus,the control 5 will operate for this blank 1 actor 23 for thepre-rounding element 14 and if the case may be as well actor 17 for therounding wedge 13, after the preceding blank 2 has left the roundingrollers 11 and 12, so that the rounding properties of the roundingapparatus are adapted to the strength properties of blank 1, so that thetarget rounding radius R is attained for blank 1 when this blank istransported through the rounding apparatus and rounded therein. The sameprocedure is done for the following blank and each of the followingblanks, so that an adaptation of the rounding properties duringoperation is attained, if necessary, for each blank. As an alternativeto the product IMPOC® another market-available product 3R-AQC of thecompany 3R Technics GmbH, Zurich, Switzerland, can be used, that allowsas well the touch-less measurement of sheet metal properties withoutdestruction of the sheet metal by generating eddy-currents within thesheet metal by a measurement coil which currents are measured. By thiseddy-current measurement and via correlation sheet metal properties suchas hardness, tear strength and elastic limit can be measured. But it ispreferred that the measurement device 27 is a device that measuresdirectly the tear limit of the sheet metal or its deformation duringrounding, respectively, as shown with the measurement device 27 of FIG.13, which measures with a sensor arrangement different touch points ofthe blank depending on its rounding properties.

In addition or in lieu of the measurement device 27 or the othermeasurement devices explained below according to FIGS. 14 to 16, ameasurement device 39 can be provided which measures the sheet metalthickness of each blank in a manner known to the skilled person. Suchsheet metal thickness measuring devices are known and available on themarket and are not explained here. The output value of the sheet metalthickness measurement device 39 is fed to the control 5 and is usedtherein to set at least actor 23 to adapt the rounding apparatus to thesheet metal property “thickness”.

With reference to FIGS. 14 to 16 a preferred embodiment for detectingthe rounding property of each blank is explained. The measurement device50 can be arranged within the feeding section 3 as explained for themeasurement devices 27 and 39. But it can as well be arranged within therounding apparatus and is then preferably a part of the flexing stationor is arranged therein. So, rollers 28 and 29 of the measurement device50 may take the place of rollers 8 and 9 of the flexing station or maytake the place of rollers in the feeding section 3. The preferred case,since less room is necessary, is that the measurement device is arrangedwithin rounding apparatus 4 and the rollers 40 and 41 are thus therounding rollers (corresponding to rollers 11 and 12 of the examplesexplained before) and thus the pre-rounding element 14 or wedge,respectively, is then arranged in front of the rounding rollers 41, 42as is shown in FIG. 14 in simplified manner by the box 14 only. As wellthe rounding wedge 13 behind the rounding rollers 41, 42 is shown by abox 13 only. Other arrangements of the measurement device 50 in front ofthe rounding apparatus or within the rounding apparatus are possible aswell. In the shown example the measurement device 50 includes a flexingwedge 37 for the measurement. If the measurement device 50 is thusarranged, as the measurement devices 27, 39, in the feeding section 3,this flexing wedge 37 can be adjusted by control 5 to the samedisplacement as the flexing wedge 7 in the rounding apparatus. If themeasurement device 50 is arranged within the rounding apparatus and inparticular within the flexing station, the flexing wedge 37 of themeasurement device will take over the function of the flexing wedge 7 ofthe rounding apparatus according to the examples above, so that therounding properties are measured with the actual flexing wedge. Themeasurement device 50 may on the other hand have no flexing wedge atall. The measurement device 50 comprises at least one sensor 45 by whichthe arrival of the sheet metal blank to be measured at or within themeasurement device can be detected. In particular the front edge (seenin transport direction) of the blank is detected, in particular by anoptical sensor, for example a light barrier or several light barriers orby an inductive sensor. This detection of the arrival of the blankstarts a time measurement by the measurement device. This timemeasurement can be done by a separate time measuring module or can bedone by control 5 which controls in this case the measurement device 50as well or is part of it. This embodiment is shown in FIG. 14. The timemeasurement will be stopped, when the front edge of the sheet metalblank hits a measuring plate 38; this is signalled to the control 5 vialine 38′. As can be seen in the side view of FIG. 14, the time isdifferent according to the rounding property of the sheet metal blankand thus the measured time is a measurement value indicating therounding property. By this value the rounding apparatus will then becontrolled for the blank as has been explained above. This is shown inFIG. 14 by signal lines 40 which lead from the control 5 to the actorsof the rounding apparatus described above. In particular actor 23 of thepre-rounding element 14 is set or controlled, respectively, in this wayby the measured rounding property.

The detection that the front edge of the blank has hit the measurementplate 38 of the measurement device 50 is done preferably electrically.This can be done by setting the plate 38 to a first electrical potentialand at least one of rollers 28, 29 to a different second electricalpotential (which applies as well to flexing wedge 37 if this wedge ispresent). If the front edge of the electrically conductive sheet metalblank hits plate 38 the two potentials are short cut which is detectableby a corresponding current flow or by a corresponding voltage drop ofthe measurement voltage. By detecting this event the time measurement isstopped or in other words the time between the detection of the arrivalof the front edge by sensor 45 and the detection of the contact of thefront edge with sensor plate 38 is taken which is dependent on therounding property of the sheet metal blank. As can be seen clearly inFIG. 14 a sheet that is less bent by rollers 29, 28 due to its bendingproperties hits plate 38 nearer to rollers 41, 42 and thus takes moretime until the contact can be detected than a blank that is bent with asmaller radius. Thus the rounding properties are detectable by themeasured time.

With coated sheet metal blanks the electrical contact between rollers28, 29 and if the case may be flexing wedge 37 and the sheet metal maybe insufficient for a measurement. It is therefore preferred that themeasuring plate is provided with a plurality of measuring parts 38 a, 38b, 38 c, 38 d etc. that extend in transport direction and are arrangedside by side and are electrically isolated from each other. Adjacentmeasuring parts are lying on the two different electrical potentials.Thus by a short cut of two measuring parts of different potential by thefront edge of the sheet metal blank, which is always uncoated, thecontact of the blank with the plate 38 can be detected as well. Thesemeasuring parts can have a wedge shape as can be seen in FIGS. 14 and15. FIG. 15 shows some of the measuring wedges in perspective view. FIG.16 shows a measurement circuit with a measurement voltage source U_(s)and where the rollers 28, 29 and the flexing wedge 37 are on groundpotential. As well being connected to ground potential are the measuringwedges 38 b, 38 d etc. (but in FIG. 16 only wedge 38 b is shown as abox). Measuring wedges 38 a, 38 c etc. on the other hand lie on positivepotential (in FIG. 16 only wedge 38 a is shown as a box). Possibleelectrical short cuts for the measurement voltage by contact of theblank front edge on the measurement plate 38 are thus the short cutmeasurement wedge—measurement wedge or measurement wedge—flexing wedge,or measuring wedge—roller. Each of these short cuts will lead to avoltage drop that can be detected and will stop time measurement.Voltage drop measurement is shown in FIG. 16 by voltage meter symbols (aV in a circle).

The method and the apparatus are in particular suited for the roundingand welding of can bodies. The pre-rounding element according to theinvention can be mounted to a rounding apparatus instead of theconventional pre-rounding element.

While there are shown and described presently preferred embodiments ofthe invention, it is to be distinctly understood that the invention isnot limited thereto but may be otherwise variously embodied andpractised within the scope of the following claims.

1. A method of rounding sheet metal blanks in a rounding apparatus inwhich each blank to be rounded is pressurized by a pre-rounding element,said pre-rounding element comprising a fixed element upper part withfixation elements for fixation to the rounding apparatus, or said upperelement part being provided by a fixed part of said rounding apparatus,a lower element part being provided for acting on the sheet metalblanks, a joint or hinge, respectively, joining said element upper partwith said element lower part, at least one actuator by which saidelement lower part is displaceable controlled by a signal and whereinsaid signal is based on a measurement of a sheet metal property of thesheet metal blanks to be rounded.
 2. A method according to claim 15wherein said sheet metal property measured is the rounding property ofsaid sheet metal blank which is measured during operation of saidrounding apparatus on consecutive blanks without destruction of saidblanks.
 3. A method according to claim 15 wherein said measurement ismade in a flexing station of said rounding apparatus.
 4. A methodaccording to claim 16 wherein said rounding property is measuredelectrically by detecting said passing blank at a first position,pre-rounding said blank and detecting said blank at a second positiondependent from the rounding and measuring the time of travel of saidblank between said first and said second position.